This invention relates to clays and, more particularly, is concerned with a process for treating clay minerals of the kandite group, i.e. clay minerals comprising kaolinite, dickite, nacrite or halloysite.
Most raw kaolinitic clays, for example, contain impurities which impair the properties of the clay for paper coating purposes and of these impurities, probably the most important ones are coarse abrasive particles (which include hard minerals such as quartz), and ferric iron-containing compounds which are dark coloured and reduce the overall brightness, or reflectance to visible light, of the clay. It is known that the effect of these discolouring impurities may be reduced by treating the clay with a reducing agent which converts ferric compounds to the less highly coloured ferrous form. The most widely used reducing agents are dithionites, such as sodium and zinc dithionite, alkali borohydrides used in conjunction with alkali bisulphite or pyrosulphite, or sulphoxylates which are compounds formed by the reaction of an aldehyde, especially formaldehyde, with metal salts of dithionous acid.
In a conventional process for reducing the ferric iron-containing impurities in a clay to the ferrous state, the clay is first suspended in water and the pH of the resulting suspension is adjusted to lie in the range of from 2.5 to 5.0. The reducing agent is then added and remains in contact with the clay suspension until substantially all of the ferric compounds which are accessible to the reducing agent (i.e. are not contained within the clay crystal lattice) are reduced to the ferrous state. The ferrous compounds are generally very soluble and pass into the water in which the clay is suspended. The pH of the suspension is then adjusted, if necessary, to about 4.5 or 5.0 and the suspension is then thickened by sedimentation, dewatered by filtration and the cake thus obtained thermally dried. At a pH in the range from 2.5 to 5.0 the clay is flocculated (i.e. the individual clay particles carry both positive and negative electric charges so that they attract one another and clump together to form `flocs`). In this state a suspension of clay ceases to be fluid when the solids content is above about 40% by weight and the process of bleaching, or reducing the ferric iron-containing impurities in the clay, is therefore usually performed at a solids content of about 15-30% by weight.
Unfortunately very few kaolinitic clays in their raw state have the type of particle size distribution which is required for a good paper coating pigment, i.e. a particle size distribution such that over 80% by weight of the particles have an equivalent spherical diameter smaller than 2 .mu.m and not more than about 3% by weight have an equivalent spherical diameter larger than 10 .mu.m. In order to achieve the desired particle size distribution it is generally necessary to perform one or more particle size separations by, for example, sieving or differential gravitational or centrifugal sedimentation. In order to perform these operations successfully it is essential that the clay should be present in the form of discrete particles rather than flocs so that the particles can be accurately sorted into different size ranges. The clay particles must therefore be treated with a deflocculant, or dispersing agent, (which will give all the particles the same electric charge--usually negative--and cause them to repel each other). Generally, particle size separations have been performed using deflocculated aqueous suspensions having a solids content of about 20% by weight or below, and it has not hitherto been considered practicable to perform a particle size separation, especially by the operations of gravitational or centrifugal separation, on a deflocculated aqueous suspension having a solids content in excess of about 60% by weight.
A paper coating pigment is generally applied to a base paper in the form of a composition comprising a deflocculated aqueous suspension of the pigment and an adhesive. The composition must be sufficiently fluid to enable it to be spread evenly over the surface of a paper web and yet must contain the minimum amount of water because all water which is added with the paper coating pigment must be evaporated thermally when the coated paper is dried, and heat energy is expensive. These two requirements can only be met if the pigment is in the deflocculated rather than the flocculated state; and a clay pigment is normally only fully deflocculated at a pH of about 7 or above. Therefore, in order to produce a good quality paper coating pigment from a raw kaolinitic clay, it is generally necessary to perform one or more particle size separations with the clay in the deflocculated state, flocculate the clay and treat the clay in the flocculated state with a reducing agent, dewater the clay in the flocculated state, and finally prepare a deflocculated suspension for paper coating. Clearly it would be advantageous if all processing could be carried out in the deflocculated state but hitherto it has not been considered possible to bleach kaolinite clays effectively with reducing bleaching agents at a high pH.
We have now made the surprising discovery that a reducing bleaching process can be carried out effectively at a pH greater than 7.0.